There has been considerable interest in recent years in advanced technology storage cells because of their potentially high energy densities per unit weight and volume. Potential applications of such cells include sources for standby power, utility load leveling, solar photovoltaic storage and vehicle propulsion. These applications may be complementary. For example, electric vehicles propelled by storage cells may contribute to utility load leveling because the storage cells can be recharged at night when utilities presently have excess generating capacity. The traditional and widely used lead-acid battery does not appear at present to be suitable for these applications. It is not practical for utility load leveling because it is unable to sustain the large number of deep discharge cycles desired, if not required, for such use. The relatively low ratio of energy density to weight, approximately 20 watt-hours/kilogram, limits, even if it does not preclude, the utility of such cells for vehicle propulsion.
Several systems have been examined as candidates for advanced technology storage cells. The systems examined have used many different materials for the electrodes and the electrolyte. One system that has been extensively investigated uses an alkali metal-sulfur couple for the electrodes. The alkali metal is typically lithium or sodium. Other alkali metals might be used, but their higher atomic weights will lower the theoretically attainable energy densities. These couples are attractive electrode candidates because of their high theoretical specific energies which are 2600 and 750 watt-hours/kilogram for the lithium-sulfur and sodium-sulfur couples, respectively.
The basic configuration of the sodium-sulfur cell has a molten metallic sodium anode, a molten sulfur cathode and a sodium ion conducting electrolyte separating the anode and cathode. In this configuration, the cell operates at a relatively high temperature. The high operating temperature, typically between 300 and 400 degrees C., is required to keep the anode and cathode materials, as well as the cathode reaction products, such as sodium polysulfides, molten. As might be expected, the high operating temperature causes complexities and problems. For example, an external power source and thermal insulation are required to keep the reactants molten. Further, both sulfur and sodium polysulfides are highly corrosive at high temperatures, and well-engineered cells require exotic structural and seal materials for safe operation. Lithium-sulfur cells have the same basic configuration as the sodium-sulfur cells.
To avoid these and other problems, much development activity has been directed toward reducing the operating temperature reange of sodium-sulfur or lithium-sulfur cells while retaining as many of the desirable attributes of the high temperature configuration as possible. However, several considerations limit the extent to which the operating temperature of sodium-sulfur cells may be lowered, and the minimum useful operating temperature for such cells is approximately 100 degrees C. This minimum is imposed by two practical considerations. Metallic sodium is not molten below 98 degrees C., and the conductivity of typical sodium ion conducting solid electrolytes is generally too low below this temperature for useful cells for the applications mentioned above.
The development activity directed toward reducing the operating temperatures of sodium-sulfur cells has involved several approaches. One approach uses catholytes in which organic solvents dissolve sulfur in cells having solid electrolytes. This approach appears to produce practical cells in the temperature range between 100 and 200 degrees C. However, the catholytes in these systems presently suffer the drawbacks of marginal solubilities for the reactants, excessive polarization and poor reversibility. These drawbacks limit capacity, rates and cycle life, respectively. Although these cells do operate at lower temperatures, their characteristics do not enable them to compete favorably with the high temperature sodium-sulfur cells at the present time.
Another approach has used cathodes in which the reactants are dissolved in molten salts. For example, one system uses cathode reactants dissolved in sodium chloride-aluminum chloride molten salts with sodium ion conducting solid electrolytes and liquid sodium anodes to construct cells operating between 175 degrees C. and 300 degrees C. However, sulfur is not very soluble in sodium tetrachloroaluminate melts. Consequently, the practical utility of this system as a battery cathode is limited. Another system using this approach is described in U.S. Pat. No. 4,063,005, issued on Dec. 13, 1977, to Gleb Mamantov and Roberto Marassi. Tetravalent sulfur is the active cathode material, and it is used in a molten chloroaluminate solvent formed by a mixture of AlCl.sub.3 and NaCl having molar ratios greater than 1:1 and less than 4:1. The tetravalent sulfur is contained in the compound SCl.sub.3 :AlClHd 4, and it is reversibly reduced to elemental sulfur and sodium chloride in the acidic molten salt solvent as the elemental sodium is oxidized on the other side of a sodium ion conducting solid electrolyte. Typical cell operating temperatures are approximately 200 degrees C.